1. Field of the Invention
Embodiments of the present invention relate, in general, to touch sensitive interfaces and more particularly to touch tracking between successive frames on a touch sensitive interface.
2. Relevant Background
A touch screen system is a computer input device/system capable of receiving input signals for a computer or processor through a pressure sensitive plate. When an input stylus, a pen, or a finger touches the pressure sensitive plate (the ‘touch screen’) at a point on the surface of the touch screen, the touch screen system senses the location of the ‘touch point’ within the area of the touch screen. The touch screen system sends information concerning the location of the touch point to the processor. The processor is operable to associate specific locations on the touch screen with certain predetermined input signals. For example, touching one area of the touch screen may instruct the computer to perform a certain function and touching another area of the touch screen may instruct the computer to perform another function.
Input devices such as a touch screen are designed to detect the application of a force and to determine one or more specific characteristics of that force as relating to the input device. These characteristics may include the location of the force as acting on the input device, the magnitude of force applied by the object to the input device, or the duration of the force.
Currently, there are a variety of different types of input devices available on the market. Some examples include resistive-based input devices, capacitance-based input devices, surface acoustic wave-based devices, force-based input devices, infrared-based devices, and others. While each provides some useful functional aspects, each of these prior related types of input devices suffers in one or more areas.
A resistance-type touch screen is generally a transparent four-layer compound screen of thin film, with the bottom being a base layer made of glass or organic glass, the top being a plastic layer whose outer surface has undergone cure process and thus becomes smooth and resistant to scratches, and the middle layer comprising two metal conductive layers disposed on the base layer and next to the inner surface of the plastic layer, respectively, the two conductive layers being spaced from each other by many minute (smaller than 1/1000 inch) transparent separating points between them. When the screen is touched with a finger, the two conductive layers contact with each other at the touch point.
The two metal conductive layers are operating faces of the touch screen, and two strips of silver paste are coated to both ends of each operating face respectively and referred to as a pair of electrodes for this operating face. If a voltage is applied to the pair of electrodes for one of the operating faces, a uniform and continuous distribution of parallel voltage will be formed on the operating face. When a prescribed voltage is applied to the pair of electrodes in the X axis direction, and no voltage is applied to the pair of electrodes in the Y axis direction, the voltage value at the touch point can be reflected on the Y+ (or Y−) electrode in the parallel voltage field along the X axis, and the coordinate of the touch point along the X axis can be obtained by measuring the voltage value of the Y+ electrode with respect to the ground. Similarly, when a voltage is applied to the pair of electrodes in the Y axis direction, and no voltage is applied to the pair of electrodes in the X axis direction, the coordinate of the touch point along the Y axis can be obtained by measuring the voltage value of the X+ electrode with respect to the ground. Finally, the coordinates of the pressure center point can be obtained by calculating a weighted average of the coordinates of all touch points with a controller for the touch screen. Unfortunately this axis histogram approach fails to reject ghost key inputs, nor can it detect multiple touches.
Most of the touch screen input modes known in the prior art have at least one thing in common: they assume that the touch-sensitive screen is touched at only one point at a time. Indeed, these screens are designed with this assumption in mind. When a user accidentally touches the screen at more than one point (for example, by hitting two ‘virtual keys’ at the same time), these screens become confused and either capture neither touch or, assuming a single touch, compute a location of the assumed single touch that is some confusing combination of the locations of the multiple touches. Either case confuses the user, and the latter case may result in unwanted input being sent to an application reading the screen.
The problem of accidentally touching more than one location at a time has existed at least since the introduction of the typewriter keyboard in the nineteenth century. Indeed the arrangement of the letters on the typewriter keyboard was designed to minimize such multiple key touches. Somewhat alleviating the problem, a user of a physical keyboard can usually tell by feel that he has hit more than one key. Unlike these physical keyboards, however, touch-sensitive screens are so rigid, and have essentially no ‘give,’ that they cannot provide tactile feedback to tell the user that he is touching the screen at more than one location.
Another problem with most touch screens is their difficulty to track the movement of multiple points. At a simple level a single touch on a screen can be moved over a series of frames to construct a track, for example a movement on a touch screen to scroll a window using the single motion (track) of a finger. However, once multiple touch points are introduced, the ability to develop independent tracks from each of these touch points becomes problematic.
One technique known in the art is to search within a certain radius of a touch point on the frame prior and on the frame after to determine whether additional points exist. If in the prior frame within a certain search distance another touch exists, a prospective trajectory can be formed. Then if in the frame after another touch point is found within the same search area and along the projected trajectory, a track can be formed. This backward and forward comparison continues until a minimal solution is achieved. Unfortunately this type of technique requires substantial computational resources and does not support new entry or exit points. For example during the period that one track has been determined, this technique does not consider the possibility that within a certain search period a new touch point may be introduced initiating a new track. Thus the challenge to efficiently track multiple points across multiple frames of a touch screen is addressed by the present invention, hereafter presented by way of example.